Re: [PATCH bpf-next 4/4] bpf: runqslower: use task local storage

[Yonghong Song <yhs@fb.com>]

On 1/8/21 3:19 PM, Song Liu wrote:
> Replace hashtab with task local storage in runqslower. This improves the
> performance of these BPF programs. The following table summarizes average
> runtime of these programs, in nanoseconds:
> 
>                            task-local   hash-prealloc   hash-no-prealloc
> handle__sched_wakeup             125             340               3124
> handle__sched_wakeup_new        2812            1510               2998
> handle__sched_switch             151             208                991
> 
> Note that, task local storage gives better performance than hashtab for
> handle__sched_wakeup and handle__sched_switch. On the other hand, for
> handle__sched_wakeup_new, task local storage is slower than hashtab with
> prealloc. This is because handle__sched_wakeup_new accesses the data for
> the first time, so it has to allocate the data for task local storage.
> Once the initial allocation is done, subsequent accesses, as those in
> handle__sched_wakeup, are much faster with task local storage. If we
> disable hashtab prealloc, task local storage is much faster for all 3
> functions.
> 
> Signed-off-by: Song Liu <songliubraving@fb.com>
> ---
>   tools/bpf/runqslower/runqslower.bpf.c | 26 +++++++++++++++-----------
>   1 file changed, 15 insertions(+), 11 deletions(-)
> 
> diff --git a/tools/bpf/runqslower/runqslower.bpf.c b/tools/bpf/runqslower/runqslower.bpf.c
> index 1f18a409f0443..c4de4179a0a17 100644
> --- a/tools/bpf/runqslower/runqslower.bpf.c
> +++ b/tools/bpf/runqslower/runqslower.bpf.c
> @@ -11,9 +11,9 @@ const volatile __u64 min_us = 0;
>   const volatile pid_t targ_pid = 0;
>   
>   struct {
> -	__uint(type, BPF_MAP_TYPE_HASH);
> -	__uint(max_entries, 10240);
> -	__type(key, u32);
> +	__uint(type, BPF_MAP_TYPE_TASK_STORAGE);
> +	__uint(map_flags, BPF_F_NO_PREALLOC);
> +	__type(key, int);
>   	__type(value, u64);
>   } start SEC(".maps");
>   
> @@ -25,15 +25,19 @@ struct {
>   
>   /* record enqueue timestamp */
>   __always_inline
> -static int trace_enqueue(u32 tgid, u32 pid)
> +static int trace_enqueue(struct task_struct *t)
>   {
> -	u64 ts;
> +	u32 pid = t->pid;
> +	u64 ts, *ptr;
>   
>   	if (!pid || (targ_pid && targ_pid != pid))
>   		return 0;
>   
>   	ts = bpf_ktime_get_ns();
> -	bpf_map_update_elem(&start, &pid, &ts, 0);
> +	ptr = bpf_task_storage_get(&start, t, 0,
> +				   BPF_LOCAL_STORAGE_GET_F_CREATE);
> +	if (ptr)
> +		*ptr = ts;
>   	return 0;
>   }
>   
> @@ -43,7 +47,7 @@ int handle__sched_wakeup(u64 *ctx)
>   	/* TP_PROTO(struct task_struct *p) */
>   	struct task_struct *p = (void *)ctx[0];
>   
> -	return trace_enqueue(p->tgid, p->pid);
> +	return trace_enqueue(p);
>   }
>   
>   SEC("tp_btf/sched_wakeup_new")
> @@ -52,7 +56,7 @@ int handle__sched_wakeup_new(u64 *ctx)
>   	/* TP_PROTO(struct task_struct *p) */
>   	struct task_struct *p = (void *)ctx[0];
>   
> -	return trace_enqueue(p->tgid, p->pid);
> +	return trace_enqueue(p);
>   }
>   
>   SEC("tp_btf/sched_switch")
> @@ -70,12 +74,12 @@ int handle__sched_switch(u64 *ctx)
>   
>   	/* ivcsw: treat like an enqueue event and store timestamp */
>   	if (prev->state == TASK_RUNNING)
> -		trace_enqueue(prev->tgid, prev->pid);
> +		trace_enqueue(prev);
>   
>   	pid = next->pid;
>   
>   	/* fetch timestamp and calculate delta */
> -	tsp = bpf_map_lookup_elem(&start, &pid);
> +	tsp = bpf_task_storage_get(&start, next, 0, 0);
>   	if (!tsp)
>   		return 0;   /* missed enqueue */

Previously, hash table may overflow so we may have missed enqueue.
Here with task local storage, is it possible to add additional pid
filtering in the beginning of handle__sched_switch such that
missed enqueue here can be treated as an error?

>   
> @@ -91,7 +95,7 @@ int handle__sched_switch(u64 *ctx)
>   	bpf_perf_event_output(ctx, &events, BPF_F_CURRENT_CPU,
>   			      &event, sizeof(event));
>   
> -	bpf_map_delete_elem(&start, &pid);
> +	bpf_task_storage_delete(&start, next);
>   	return 0;
>   }
>   
>